The present invention relates to a method for constructing a tunnel in a formation under hydraulic pressure, in which method an underground gallery is driven by mechanical rock excavation, the excavated rock is removed and the gallery is sealed against the hydraulic pressure.
Hydraulic pressures, especially high hydraulic pressures approaching about 130.times.10.sup.5 Pa, as are expected to exist at the level of the planned Gotthard base tunnel in the Piora Syncline, for example, represent major problems for tunnel construction. This is further exacerbated when the formation, as in the cited example, is difficult to compact and has poor stability. Hydraulic pressure represents an enormous threat to miners, since in the worst case an inrush of water can very rapidly inundate the gallery. Lowering the hydraulic pressure to alleviate this danger may be extremely difficult, depending on the permeability of the problem formation zone, and it represents an enormous disturbance of the natural hydraulic equilibrium of the formation, with unforeseeable results.